This invention relates generally to quartz processing and more particularly to the production of high purity quartz (BPQ) by evaporatively reacting preheated quartz in an acid solution to cool the quartz while at the same evaporating the liquid from the solution and reacting the acid with the quartz impurities to form soluble salts.
Natural quartz is often found in the presence of minerals such as feldspar, mica, kaolin, and garnet. Various means of mineral separation are normally employed to produce commercial grade products of each of these minerals, including industrial sand products as well. Such mineral processing steps are well known and are briefly described in U.S. Pat. No. 5,037,625. While such processing will provide gross separation of the quartz from the in-situ minerals of the deposit, the resultant sand product is usually only clean enough for construction or landscaping applications. Further processing is required to remove impurities still associated with the quartz particles, both on the surface and within the structure of the crystal itself, as well as other inclusions that cause difficulties in fusion of the quartz, in order for the purity of the quartz particles to be used in the semiconductor and high-intensity lighting industries.
An essential component to the semiconductor and high-intensity lighting industries is crystalline high purity quartz (silicon dioxide, SiO.sub.2), commonly referred to as HPQ. HPQ is used for the manufacturing offused quartz products in both industries and is typically characterized by the low level of impurities present; these impurities include Al, Ca, Na, Mg, Fe, Ti, Li, K, and B.
The semiconductor industry is the largest user of fused quartz, producing wafers (integrated circuit substrates) and quartzware furniture (various accessories used in wafer manufacturing). The purity requirements for semiconductor processing are quite stringent with total impurity limits of 15 parts per million (ppm) or less for the purest HPQ products. Fused quartz is also used in the lighting industry primarily for bulb envelopes for high-intensity lights such as halogen, metal hydride, and mercury vapor lamps. This market has recently seen rapid expansion and is predicted to remain strong due to the halogen headlight requirement on more than eighty-five percent of new vehicle production. The purity requirements for fused quartz glassware are not, however, as demanding as those for semiconductor use, tolerating a total impurity level as high as 30 ppm.
An equally important parameter of the BPQ product is its melting or fusing characteristics. High purity granular quartz can be unsuitable for fused quartz production due to the presence of excessive gas and/or liquid inclusions or pockets distributed throughout the crystal. These inclusions are believed to be responsible for bubble formation in fused quartz; and excessive bubble content, as with excessive impurity content, is unacceptable for fused quartz in semiconductor and lighting applications.
Prior art techniques which have been employed for removal of surface bound impurities and reduction of bubble forming inclusions, both by producers of BPQ and users of HPQ who process low purity quartz feed, include leaching (caustic or acidic) and high temperature treatment with various acid contact to convert the impurities to soluble salts. These prior art processes are exemplified in the following patents:
Patent No. Inventor Issue Date Class/Subclass 2,182,384 McGregor 12/1939 423/340 2,493,304 McCready et al. 1/1950 423/340 3,666,414 Bayer 5/1972 423/340 4,804,422 Papanikolav et al. 2/1989 423/340 4,818,510 Jung 4/1989 423/340 4,956,059 Englisch et al 7/1990 423/340 4,983,370 Loritsch et al 1/1991 423/340 5,037,625 Loritsch et al. 9/1991 423/340 5,637,284 Sato et al. 1/1997 423/430
U.S. Pat. No. 4,804,422 describes the purification of sand by means of acid washing in which at least 40% of the initial weight of the sand is dissolved.
Impurity removal from within the structure of the crystalline lattice of the quartz, however, requires interstitial access that is generally not possible with external leaching without the dissolution or etching of the quartz from the strong acidic or basic washing. U.S. Pat. No. 4,818,510 discloses the method of heating to at least 1650.degree. C. and quenching (as defined in the patent as the introduction into an environment at least 200.degree. C. cooler) in an attempt to fracture the quartz particles and expose occluded and interstitial impurities. The fractured quartz particles can then be treated with further leaching and heat treatment steps. This method, used in conjunction with selective sizing, has proven inadequate to achieve the very low impurity levels required for the most stringent of semiconductor specifications.
U.S. Pat. No. 2,182,384 describes the recovery of sand from grinding debris resulting from glass grinding and polishing including treatment with chlorine gas at temperatures of 1400-1800.degree. F. U.S. Pat. Nos. 4,983,370 and 5,037,625 teach contacting surface-cleaned quartz with HCl gas at a temperature of 800.degree. C. to 1600.degree. C. for a duration of several minutes to as long as several hours. These methods are designed to diffuse the impurities to the quartz surface to form chloride salts which can then be removed. U.S. Pat. No. 2,493,304 describes a furnace for treating quartz by cascading the quartz over a series of discs while countercurrent flowing air and chlorine gas through the cascading quartz in order for the chlorine to react with the quartz impurities and the air to carry away the reaction products leaving reduced impurity quartz.
U.S. Pat. No. 3,666,414 discloses the treatment of rock crystal to produce fused silica which includes treatment with silicon tetrachloride followed by extended heating to drive off the hydrogen chloride gas and drying under vacuum.
U.S. Pat. No. 4,956,059 discloses a process wherein the quartz is heated to a range of 700-1300.degree. C. in a rotary kiln, the kiln rotated to thoroughly mix the heated quartz while gaseous chlorine or hydrogen chloride is passed through the chamber. After chlorine treatment, the treated quartz is allowed to rest for at least ten times longer than the treatment period while an electric field of 600-1350 volts is imposed across the chamber. This process is repeated several times during the treatment of the quartz. U.S. Pat. No. 5,637,284 discloses a continuous process utilizing high temperature reaction in a chlorine-containing gas atmosphere, followed by a gas desorption step.
High temperature chlorination has proven to be the most effective final processing step to date to achieve the purest HPQ product from natural crystalline quartz. Prior art methods utilizing chlorination are expensive and limited to the use of hydrogen chloride, primarily due to the requirement of prolonged thermodynamic stability at 800.degree.-1600.degree. C.